According to a conventional sensor controller system, the state of the articles which are being continually conveyed by a conveyer belt or the like is detected by using a plurality of sensors. The sensors may detect the spacing between articles such as bottles conveyed by the conveyer belt, position of a label on each of the bottles, and so on. In such a sensor controller system, a plurality of sensors such as photoelectric sensors are used to detect the conditions of the articles themselves and the way they are being conveyed.
As these sensors typically consist of reflection type and transmission type photoelectric sensors, there is a possibility that any one of the sensors becomes incapable of functioning properly. For instance, deposition of dust and other foreign matters on light emitting or receiving surfaces would cause reduction in the amount of light emitted or received. The axes of light beams may deviate from prescribed paths due to mechanical failures, and sensor elements themselves may become inoperative as a result of various electric failures. Therefore, it is desirable to provide means for monitoring the soundness of sensors so that any failures may be corrected without causing costly delays.